Process and apparatus for oxidizing industrial spent caustic and effecting gas-liquid mass transfer and separation

ABSTRACT

An improved process for oxidizing industrial spent caustic streams, whereby oxidation of spent caustic occurs in a unitary processing tower with three chambers that cooperate simultaneously to oxidize the caustic in the first chamber; to separate the caustic from the oxidizing gas in the second chamber; and, in the third chamber, to pass gas and cooled caustic through a mass transfer apparatus that cools and cleans the gas and releases the caustic and gas into a gas-liquid separation zone of the upper chamber. An alternative embodiment of the invention involves spatially separating the upper chamber from the other two chambers.

BACKGROUND OF THE INVENTION

This invention relates to the oxidation of the spent caustic occurringin the process of desalting crude oil.

Among the most common sent caustics produced by industry are thosecontaining sodium sulfide, sodium bisulfide, sodium bisulfide, sodiumnaphthenates, and sodium cresylates. If the above spent caustics aredischarged directly into plant effluent waters, they place a burdensomeoxygen demand on the receiving waters. Fish and other aquaticpopulations are adversely are adversely affected, and water may becomeunsafe for human consumption or it may gain a distasteful flavor. Forthese and other reasons, spent caustics are normally converted to a formthat minimizes these problems.

Systems for oxidizing these spent caustic have been around for manyyears, but such systems have required complex, expensive multi-vesselfacilities. For example, the Shell design described in U.S. Pat. No.2,869,844, "Treating Liquid With Gas", Issued Jan. 20, 1959, by S. B.Thomas, incorporated by reference herein, and also described in anarticle by J. D. Martin and L. D. Levanas, "Air Oxidation of Sulfide InProcess Water", Division or Refining, Vol. 42, pp. 392-98 (1962),required a feed/product exchanger for a gas/liquid mixture and threeprocessing vessels: an oxidation column, a vessel for gas/liquidseparation, and an offgas furnace. A flow diagram of the Shell processis provided in the article as FIG. 1 on page 393. A similar system isdescribed by Milton R. Beychok in Aqueous Wastes From Petroleum andPetrochemical Plants, (2nd ed. 1973 John Wiley & Sons 1973) in a chapterentitled "Treatment Methods," pp. 156-266. On page 202, in particular,FIG. 30 shows a diagram of a typical oxidizing unit which issubstantially the same as the Shell design. The chapter of this bookentitled "Treatment Methods" is incorporated herein as if fully set outverbatim. Merichem Company also previously designed systems requiting afeed/product heat exchanger for a gas/liquid mixture and three vessels:an oxidation tower, an offgas K.O. drum for gas/liquid separation, andan offgas treater. This is described in F. J. Suarez, "Safe WasteCaustic Disposal", Hydrocarbon Technology International '89/'90, whichis incorporated herein by reference. The Suarez article contains twoflow diagrams as FIGS. 1 and 2 that show the previous equipment andprocess required for oxidation of spent caustic. FIG. 1 includes aneutralization system, as well, for adjusting the pH of oxidizedcaustic. The offgas treater is not shown in either figure. Otheroxidation processes have been developed that are even more complex. Forexample, in U.S. Pat. No. 4,384,959, Baurer, et at, "Wet OxidationProcess Utilizing Dilution of Oxygen", issued May 24, 1983, a method forwet oxidation of aqueous liquor containing combustible matter isdescribed which requires a separate vessel for gas-liquid separation; apressure control valve to reduce the pressure of cooled oxidized liquorand offgases; a feed/product exchanger; and means for injection of inertgas into the waste liquor to dilute the oxygen in the gas phase. Anotherexample appears in U.S. Pat. No. 4,812,243, Brandenburg, et al.,"Process for Treating Caustic Metal Wastes", issued Mar. 14, 1989,incorporated herein by reference, which describes a method for treatingcaustic cyanide and metal waste by wet air oxidation.

Thus, a system for oxidizing caustic that is simpler to build andoperate, and eliminates use of a costly feed/product mixed phaseexchanger to cool the oxidized caustic and gas released from theoxidation tower is needed in the industry. Furthermore, an advancementover the present technology, where caustic and hot gas would beseparated before being transported from the oxidizing tower would beadvantageous because it would eliminate special metallurgical andsurface area requirements for downstream equipment such as an exchanger.F. J. Suarez, "Safe Waste Caustic Disposal", Hydrocarbon TechnologyInternational '89/,90, p. 2 of the reprinted article.

SUMMARY OF THE INVENTION

The present invention is directed to a new and improved method andapparatus for oxidizing industrial spent caustic streams, wherebyoxidation of caustic occurs in a unitary processing tower with threechambers that cooperate simultaneously to oxidize the caustic in thefirst chamber; to separate the caustic from the oxidizing gas in thesecond chamber; and, in the third chamber, to pass gas and cooledcaustic through a mass transfer apparatus that cools and cleans the gasand then releases the caustic and gas into a gas-liquid separation zoneof the upper chamber. An alternative embodiment of the inventioninvolves spatially separating the upper chamber from the other twochambers.

The present invention results in an efficient compact system foroxidizing industrial spent caustic. Among its many advantages, otherthan those mentioned above, is the use of one or two processing vesselsinstead of three vessels, as required by prior oxidation systems.Separate vessels are not required for gas-liquid separation and offgascleaning. Another advantage of the present invention is the eliminationof the costly mixed phase feed/product exchanger required in earliersystems. The present method, instead separates the gas and caustic inthe second and third chambers prior to transporting the gas and causticto their next destination, allowing for use of caustic/cooling waterexchangers for cooling the caustic. This eliminates the surface area andspecial metallurgical requirements of the mixed phase feed/productexchangers. An additional advantage is the mass transfer apparatus inthe third chamber that cools the gas before offloading it, and moreover,removes odorous compounds from the offgas, so that the offgas can bereleased to the atmosphere. This eliminates the need for a downstreamoffgas scrubber. Yet another advantage of the present invention is thecompact unitary tower arrangement, which minimizes piping and requiredplot area. For all of the above reasons, less equipment is required forthe method and apparatus of this invention than with methods andapparatuses known to the art. Equipment investment and costs ofoperation, including engineering manhours, are reduced substantially.

It is an object of the present invention to provide an improved processfor oxidizing industrial spent caustic whereby the improvement comprisesoxidizing spent caustic in a lower chamber of a unitary processing towerthat provides an oxidation zone where the spent caustic is contactedwith an oxidizing gas, preferably air. Then, the caustic and gas arecommunicated from the lower chamber to a middle chamber through achimney tray defining the upper limit of the lower chamber. The gas andcaustic separate by gravity in a separation zone of the middle chamber,where the caustic collects as a lower phase in the chimney tray and thegas collects as an upper phase. The collected gas is then transportedthrough a conduit to an upper chamber. In one preferred embodiment, theupper chamber is affixed to the middle chamber. In an alternativeembodiment, the upper chamber is spatially separated from the middlechamber. Meanwhile, the oxidized caustic is transported into a lowerrecycle conduit with means for cooling the caustic before transportingit to the upper recycle conduit, the lower chamber, or to an offloadingconduit. Gas from the middle chamber and cooled caustic from the upperrecycle conduit are then introduced to the upper chamber through a masstransfer apparatus that effects contact between the gas and cooledcaustic and effects cleaning of the gas. The gas and caustic thenseparate by gravity in a separation zone of the upper chamber wherecaustic collects in the bottom of the chamber as a lower phase and gascollects as an upper phase. The resulting gas is then offloaded to theatmosphere or to any other desired destination. Caustic collected in theupper chamber is transported into the upper recycle conduit.

It is still another object of the present invention to provide animproved apparatus for oxidizing industrial spent caustic usinggas-liquid mass transfer and gas-liquid separation, all in a unitaryprocessing tower. The unitary processing tower comprises a lower,middle, and upper chamber, where the lower chamber provides an oxidationzone wherein the spent caustic is contacted with an oxidizing gas,preferably air. The lower chamber has means for introducing the spentcaustic into the chamber adjacent to the bottom of the chamber; meansfor introducing the gas into the chamber to contact the caustic near thebottom of the chamber; and a chimney tray defining the upper limit ofthe lower chamber. The middle chamber forms a gas-liquid separation zonecommunicating with the lower chamber through the chimney tray, where thechimney of the tray defines a liquid collection zone in the lower partof the middle chamber with a gas phase zone above the liquid collectionzone. The middle chamber also has means for removing liquid from theliquid collection zone and means for removing gas from the gas phasezone. The upper chamber comprises means for introducing the cooledoxidized caustic and the gas to the chamber, said means providing forcontact between the caustic and gas to allow for cooling and cleaning ofthe gas; a gas-liquid separation zone; means for removing the collectedcaustic from the chamber; and means for removing the gas from thechamber. In one preferred embodiment, the upper chamber is affixed tothe middle chamber. In an alternative embodiment, the upper chamber isspatially separated from the middle chamber.

The unitary processing tower of the present invention also includes aspent caustic conduit in communication with the lower chamber forintroducing the spent caustic into the chamber for oxidation; a secondcaustic conduit communicating with the lower chamber for introducingcooled caustic to the chamber; a gas conduit communicating with thelower chamber to introduce the oxidizing gas into the spent caustic; aconduit connecting the means for removing the gas from the middlechamber to the means for introducing the gas to the upper chamber; aconduit in fluid communication with the liquid collection zone of themiddle chamber through the means for removing liquid; a conduitcommunicating with the upper chamber for introducing cooled caustic intothe chamber; and two recycle conduits in communication with the causticconduits. In addition, the upper chamber includes a conduit in fluidcommunication with the means for removing liquid from the upper chamber,and a gas conduit in communication with the means for removing cooled,cleaned gas from the upper chamber. Further, the apparatus has means forcooling liquid in each of the recycle conduits.

In one preferred embodiment of the invention, the oxidizing zone isdivided into multiple compartments, vertically superposed in the lowerchamber, each forming an oxidation zone as the caustic and gas travelthrough each compartment. Further, the means for cooling the caustic inthe two recycle conduits uses water as the cooling medium. In addition,the fiber bundle mass transfer apparatus in the upper chamber comprisesa conduit extending into the upper chamber having inlets for introducingthe gas and caustic and an outlet for release of the gas and causticfrom the conduit, the conduit containing a plurality of fiberspositioned longitudinally within the conduit making contact with thecaustic collected in the upper chamber.

Still other objects, features and advantages of the present inventionwill be apparent from the following description of the preferredembodiments, given for the purpose of disclosure, and taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow diagram showing the preferred embodiment ofthe apparatus employed in the practice of this invention.

FIG. 2 is a partial schematic flow diagram of an alternative embodimentof the apparatus, wherein the upper chamber is spatially separated fromthe rest of the processing tower.

FIG. 3 is a schematic flow diagram showing an alternative preferredembodiment of the process servicing both recycle loops with a singlecaustic/cooling water exchanger.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, FIG. 1 shows a preferred embodiment utilizinga unitary processing tower 2 having a lower chamber 4 providing anoxidation zone 6 wherein the spent caustic is contacted with anoxidizing gas. The lower chamber 4 has an inlet 8 for introducing thespent caustic into the chamber adjacent to the bottom of the chamber.This inlet may be a simple port, an injector, or other means commonlyknown in the industry. The lower chamber 4 also has means 10, usually asparger, for introducing the gas uniformly into the chamber to contactthe caustic near the bottom of the chamber. A chimney tray 12 separatesthe lower chamber 4 in the preferred embodiment and middle chamber 14housing a gas-liquid separation zone 16. The lower chamber 4communicates with middle chamber 14 through the chimney tray 12 by wayof the chimney 12a which, in conjunction with tray 12, defines a liquidcollection zone 18 in middle chamber 14, with a gas phase zone 20 abovethe liquid collection zone 18. In an alternative embodiment of theinvention, the chimney tray 12 may be replaced by a conduitcommunicating from the lower chamber 4 to the middle chamber 14. Themiddle chamber 14 also has an outlet 22 for removing caustic from theliquid collection zone 18 and an outlet 24 for removing gas from the gasphase zone 20. The outlets for removing the caustic and the gas from themiddle chamber 14 may be ports or other means well known in theindustry.

The unitary processing tower 2 also has an upper chamber 26 foreffecting contact between the gas removed from the middle chamber 14 anda stream of cool oxidized caustic recycled from the middle 14 and upper26 chambers. The upper chamber 26 has means 30 for introducing thecooled oxidized caustic and the gas to the chamber which provides forcontact between the caustic and gas to allow for mass transfertherebetween and cleaning of the gas, removing certain contaminants andthe like in the gas.

In a preferred embodiment of the invention, the means 30 for introducingthe cooled oxidized caustic and gas is a mass transfer apparatus,well-known in the industry, and any such apparatus that effects intimatecontact between a liquid and a gas and will fit in the tower 2 may beused. In the preferred embodiment of the invention, fiber bundle similarto those disclosed in the following two patents is used: U.S. Pat. No.3,992,156, K. E. Clonts, "Mass Transfer Apparatus", issued Nov. 16,1976, and U.S. Pat. No. 3,997,829, K. E. Clonts, "Liquid-Liquid MassTransfer Apparatus", issued Aug. 31, 1976. Similar mass transferapparatuses were also disclosed in U.S. Pat. No. 4,666,689, R. E. Maple,et al., "Process for Regenerating an Alkaline Stream ContainingMercaptan Compounds", issued May 19, 1987, and U.S. Pat. No. 4,675,100,R. E. Maple, et al., "Treatment of Sour Hydrocarbon Distillate", issuedJun. 23, 1987. All four of these patents are incorporated fully hereinby reference as if set out verbatim. The mass transfer apparatus of thepreferred embodiment comprises a conduit 31 extending into the upperchamber 26 having inlets 33a, 33b, for introducing the gas and causticand an outlet 33c for release of the gas and caustic from the conduit31. The conduit 31 contains a plurality of fibers 35 positionedlongitudinally within the conduit 31 to allow a downstream end 35a ofthe fibers 35 to extend out of the conduit 31 making contact withoxidized caustic in a liquid collection zone 28a of the upper chamber26.

After introduction through means 30, the gas and caustic separate in agas-liquid separation zone 28. The upper chamber also has means 32 forremoving the oxidized caustic from the chamber 26 and means 34 forremoving the gas from the chamber 26. The means for removing the causticand gas from the upper chamber 26, in one preferred embodiment, areports, but other means well known in the industry may be used.

Various conduits communicate with the unitary processing tower 2 in thepreferred embodiment. The lower chamber 4 has a spent caustic conduit 36that introduces caustic into the chamber 4. A second caustic conduit 38communicates with the lower chamber 4 to introduce cooled caustic to thechamber 4 in response to temperature controller 62 to maintain operatingtemperatures selected for the operation. This second caustic conduit 38has a temperature control valve 62 connected therein that operates valve62a to adjust flow of cooled caustic into the lower chamber 4 from thelower recycle conduit 49b, when gas and caustic in the lower chamber 4reach above a specified temperature level, which is in the range ofabout 180° F. to about 225° F. in the preferred embodiment. A gasconduit 40 communicates with the lower chamber 4 to introduce oxidizinggas to the spent caustic through the means 10 for introducing gas to thelower chamber 4. This means 10 is connected to a flow distributor 52positioned in the bottom of the lower chamber 4 in one preferredembodiment. The flow distributor 52 may be any number of devicescommonly known in the industry, but is a perforated pipe in thepreferred embodiment.

In addition, a gas conduit 42 connects the middle chamber 14 to theupper chamber 26 to transfer gas from the middle chamber 14 to the upperchamber 26. The middle chamber 14 also has a conduit 44 in fluidcommunication with the liquid collection zone 18 of the middle chamber14 through the outlet 22 for removing hot liquid oxidized caustic fromthe middle chamber 14.

Cooled caustic is introduced to the upper chamber 26 through a conduit45 communicating with that chamber. Further, liquid conduit 46 and gasconduit 48 transport caustic and gas, respectively, from the upperchamber 26. In one preferred embodiment this gas conduit 48 has a backpressure control valve 58 connected therein for controlling thetransport of gas from the upper chamber 26. The exit means 34 forremoving gas from the upper chamber 26 includes a alemister pad 34a anda second conduit 59 controlled by a pressure relief valve 60 for removalof gas for safety purposes in over-pressure situations. The demister pad34a, also known as an impingement separator, is well known in theindustry and commercially available from ACS Industries Inc. and theKOCH Company. Demister pads are commonly made of stainless steel and ofa thickness in the range of about 6 inches to about 2 feet. The demisterpad has a thickness of about 6 inches in the preferred embodiment.

Conduits 45 and 46 are part of an upper recycle loop 49a and conduits 38and 44 are part of a lower recycle loop 49b which circulate caustic,usually at a substantially constant flow rate, through the unitaryprocessing tower 2. This is discussed in more detail below in connectionwith the preferred embodiment of the invention. The upper and lowerrecycle loops 49a, 49b have cooling means 50 for cooling causticcirculating through the conduits. In a preferred embodiment of theinvention, this cooling means 50 is one or more caustic/cooling waterexchangers. These heat exchangers, using water as the cooling mediumreplace prior art heat exchangers where the hot two phase stream wasused as a heat exchange medium.

Of course, the construction of and interconnections between the recycleloops communicating with the unitary processing tower 2 may be variedconsiderably by one skilled in the art. For example, the causticconduits 38, 44, 45 and 46 may all be connected into one recycle loop 49with one or more caustic/cooling water exchangers 50. This embodiment ofthe present invention is shown in FIG. 3 and could also be builtaccording to the embodiment shown in FIG. 2, where the upper chamber 26is separated from the middle chamber 14. In this embodiment, thecaustic/cooling water exchanger 50, if only one is used, may be a doublepass exchanger.

The middle chamber 14 includes a liquid level controller 54 thatregulates the release of oxidized caustic from the lower recycle loop 49into a caustic offloading conduit 55 in response to the caustic level inthe middle chamber 14. The upper chamber 26 also includes a liquid levelcontrol system 56 that allows, as approaching a control point in thechamber 26, release of cooled oxidized caustic from the upper 49a to thelower recycle conduit 49b is varied according to the nearness to thepoint and rate at which it changes.

Additionally, the lower chamber 4 preferably is divided into multiplecompartments 64 vertically superposed in the chamber 4, each forming anoxidation zone 6. These compartments 64 have means 66 for communicatingthe gas and spent caustic up the unitary processing tower 2 through thecompartments 64. The openings between the compartments serve also tohelp maintain a functioning two phase system to achieve the contactnecessary to successful oxidation. Selection of the number ofcompartments, while purely arbitrary, may be varied depending upon thedesired oxidation efficiency of the tower 2, feed rates, level ofimpurities and even the dimensions of the tower 2. In the embodimentillustrated, the lower chamber 4 is divided into four compartments 64.The means 66 for communicating the gas and spent caustic through thecompartments 64 comprises a redistributor much like sparger 52positioned in each partition 68 separating two compartments 64. Thesecond caustic conduit 38 communicating with the lower chamber 4connects thereto at the second compartment 64 from the bottom of thechamber 4.

An alternative embodiment of the invention, as shown in FIG. 2,comprises the same unitary processing tower 2, except that the upperchamber 26 is spatially separated from the body of the middle chamber14. In FIG. 2, the numbered components for the alternative embodimentare the same as the components in FIG. 1 because the same equipment isused, the only difference being the separation of the upper chamber 26from the middle chamber 14. FIG. 2, however, is a partial schematicdiagram of the alternative embodiment of the apparatus to the extentthat it does not show all of the conduits and the caustic/cooling waterexchangers 50 shown in FIG. 1. However, those conduits and theexchangers 50 are part of the alternative embodiment. When the upperchamber 26 is separated, it may be placed above or next to the rest ofthe tower 2.

In the preferred embodiment, where the middle and upper chambers are notseparated, the partition 70 is built thicker to accommodate thetemperature differences between the middle 14 and upper 26 chambers andto accommodate the properties of the caustic and gas. Alternatively, thepartition 70 may be made of a material that can withstand largertemperature differentials. The upper chamber 26, in the preferredembodiment, is at a temperature range of about 110° F. to about 120° F.in its liquid collection zone 28a. The temperature of the middle chamber14 in the gas phase zone 20 is in the range of about 180° F. to about225° F. However, these temperature ranges may vary greatly dependingupon the size and other operating parameters of the apparatus, as willbe discussed below. The unitary processing tower 2 is made of stainlesssteel in the preferred embodiment. However, the tower 2 may have a widerange of dimensions and may be made of any suitable material that canendure the temperature, pressure, and other operating conditions of thesystem, including, but not limited to, other nickel-chrome based alloys.Materials suitable for partition 70 include, but are not limited to,stainless steel or other nickel-chrome based alloys. Both materials anddimensions of the tower 2 are matters of engineering design.

The preferred method of this invention is an improved process foroxidizing industrial spent caustic in a processing tower of the typewherein an oxidation zone chamber provides contact between the spentcaustic and oxidizing gas. The improvement comprises oxidizing the spentcaustic in a lower chamber 4 of a unitary processing tower 2 whichprovides an oxidation zone 6 wherein the spent caustic is contacted withan oxidizing gas, preferably air. The spent caustic and gas are thencommunicated from the lower chamber 4 to a middle chamber 14 through achimney tray 12 defining the upper limit of the lower chamber 4.Alternatively, the spent caustic and gas may be communicated to themiddle chamber 14 through a conduit. The spent caustic and gas are thenseparated by gravity in the middle chamber 14, where the spent causticcollects in the chimney tray 12 and the gas collects above the caustic.The gas is then transported from the middle chamber 14 through a conduit42 to an upper chamber 26. The spent caustic is transported from themiddle chamber 14 into a lower recycle loop 49b with means 50 forcooling the spent caustic before it is transported to one of threedestinations: the upper recycle loop 49a, the lower chamber 4, or anoff-loading conduit 55.

Caustic transported to the lower chamber 4 from the lower recycle loop49b may be mixed with the spent caustic feed stream prior to enteringthe chamber 4. A static in line mixer may be used for such mixing.Furthermore, if the temperature of the spent caustic entering thechamber 4 is not high enough, steam may be mixed with the caustic streambefore the caustic enters the lower chamber 4 through line 36. In onepreferred embodiment, a temperature control is used to monitor thetemperature of the caustic feed stream after recycle caustic has beenmixed into it. Then, if the feed stream temperature is too low, thetemperature control system releases steam into the caustic and may alsoopen a valve to allow bypass around the first caustic/cooling waterexchanger 50 in the lower recycle loop 49b.

The gas and cooled caustic from the upper recycle loop 49a areintroduced next into the upper chamber 26 through a mass transferapparatus 30 that effects contact between the gas and cooled spentcaustic and cleans the gas. The spent caustic and cooled gas are thenallowed to separate by gravity in a gas liquid separation zone 16 in theupper chamber 26. The clean gas from the upper chamber 26 is transportedout of the chamber to a conduit 48 for offloading, and the caustic istransported from the upper chamber 26 into the upper recycle loop 49a.The gas, however, passes through the demister pad 34a before beingoffloaded into gas conduit 48. Because the gas is cleaned in the masstransfer apparatus 30, it does not have to subsequently pass through anoffgas treater before being offloaded.

When spent caustic and gas are introduced into the lower chamber 4, inthe preferred process they make contact in the chamber adjacent to thebottom of the chamber 4. One preferred process of the invention includesthe additional step of using a flow distributor 52 to mix the gas withthe spent caustic in the lower chamber 4.

The step of oxidizing the caustic in the lower chamber 4 includes, in apreferred process of the invention, passing the caustic and gas throughmultiple compartments 64 vertically superposed in the chamber 4, eachforming an oxidation zone 6. The preferred number of compartments 64 isfour, but the number may be varied depending upon the desired oxidationefficiency. The gas and caustic are then passed from compartment 64 tocompartment 64 through a redistributor 66 positioned in each partition68 separating two compartments 64.

The amount of spent caustic in the middle chamber 14 may be monitoredand adjusted by a liquid level control system 54. When liquid level ofthe middle chamber 14 changes, the liquid level control system 54adjusts flow of caustic from the lower recycle loop 49b into anoffloading conduit 55.

Communicating the gas and caustic through the mass transfer apparatus 30preferably involves using a mass transfer apparatus 30 with a conduit 31extending into the upper chamber 26 having inlets 33a, 33b forintroducing the gas and caustic, respectively, and an outlet 33c forrelease of the gas and caustic from the conduit 31. This conduit 31contains a plurality of fibers 35 positioned longitudinally within theconduit 31 making contact with collected caustic in a liquid collectionzone 28a of the upper chamber 26.

A liquid level control system 56 preferably monitors the caustic levelin the upper chamber 26. When a set capacity of the upper chamber 26 isreached, the control system 56 releases caustic from the upper recycleloop 49a into the lower recycle loop 49b. Gas is normally transportedfrom the upper chamber 26 through gas conduit 48 controlled by a backpressure control valve 58. Gas may also be released from the upperchamber 26 through a second conduit 59 controlled by pressure reliefvalve 60 for safety purposes in over-pressure situations. A gas releasethrough valve 60 occurs when a critical level of pressure occurs in theupper chamber 26. The critical level of pressure required in thepreferred embodiment to release gas through conduit 59 is in the rangeof about 125 to about 150 psig. Gas is transported through a alemisterpad 34a in the upper chamber 26 prior to release into gas conduit 48 orthe second conduit 59 for removal of gas, as discussed above.

In the preferred process, caustic in the upper and lower recycle loops49a, b passes through caustic/cooling water exchangers 50 incommunication with each conduit. The upper recycle loop 49a preferablyhas one caustic cooling water exchanger, while the lower recycle loop49b has two exchangers.

Of course, caustic may be transported from chamber to chamber of theunitary processing tower 2 through a variety of interconnections betweenthe caustic conduits communicating with the tower 2, as would be obviousto one skilled in the art. As mentioned above, the caustic conduits 38,44, 45 and 46 may all be connected into one recycle loop 49 with one ormore caustic/cooling water exchangers 50 (as shown in FIG. 3). Insofaras possible the numbering system is consistent across all three figures.

Additionally, when the gas and caustic in the lower chamber 4 reach aspecified temperature, a temperature control valve 62a opens and allowsmore cooled caustic to flow into the chamber 4 from a second conduit 38that is connected to the lower recycle loop 49b. The specifiedtemperature, in the preferred embodiment, required for openingtemperature control valve 62 is in the range of about 180° F. to about225° F.

The unitary processing tower 2 of the present invention may be operatedunder a wide range of conditions, including temperatures ranging fromabout 150° F. to about 400° F. Operating pressures may range from about30 psig to about 500 psig.

In the preferred embodiment, the spent caustic feed stream enters thelower chamber 4 at about 180° F. Steam that is at about 353° F. andabout 125 psig is injected and mixed with the spent caustic feed streamprior to entering the lower chamber 4 on an as needed basis to raise thetemperature of the spent caustic. Oxidizing gas, which is preferablyair, is injected to the lower chamber 4 at a temperature of about 80° F.and about 100 psig. The caustic waste and gas reach a temperature ofabout 180° F. to about 225° F. after undergoing the exothermic oxidationreaction in the lower chamber 4. After gravity separation in the middlechamber 14, hot caustic enters the lower recycle loop 49b where some ofthe caustic is partially cooled by one caustic/cooling water exchanger50 to a temperature in the range of about 180° F. to 185° F. Part ofthis caustic is transported back to the beginning of the process whereit is mixed with the spent caustic feed stream and cycled through thetower 2 again. The rest of the caustic is passed through a secondcaustic/cooling water exchanger 50, where it is cooled to a temperatureof about 100° F. Part of this cooled caustic is then offloaded throughconduit 55 and part of it passes to the upper recycle loop 49a. Hot gas(at a temperature of about 180° F.-225° F.) is transported from themiddle chamber 14 to the upper chamber 26 where it is contacted by coolcaustic (at a temperature of about 100° F.) from the upper recycle loop49a. After the caustic and gas pass through the mass transfer apparatus30, the gas and caustic reach a temperature in the range of about 110°F. to about 120° F. Some additional operating temperatures and pressureswere described previously above.

Common engineering elements such as pumps, gauges, valves, controllersand the like are not shown or described except when necessary for theunderstanding of the invention since for the most part selection andplacement of such equipment is well within the skill of the ordinaryengineer. Although the above process and apparatus are described interms of the above preferred embodiments, those skilled in the art willrecognize that changes in the process and apparatus may be made withoutdeparting from the spirit of the invention. Such chances are intended tofall within the scope of the following claims.

What is claimed is:
 1. In the process of oxidizing industrial spentcaustic in a processing tower of the type wherein an oxidation zonechamber provides contact between the spent caustic and an oxidizing gas,the improvement comprising:oxidizing the spent caustic in a lowerchamber of a unitary processing tower providing an oxidation zonewherein the spent caustic is contacted with an oxidizing gas;communicating the caustic and gas from the lower chamber to a middlechamber; separating the spent caustic and gas in the middle chamber,wherein the caustic collects in the bottom of the chamber and the gascollects above the caustic; transporting the gas from the middle chamberthrough a conduit to an upper chamber; transporting the caustic from themiddle chamber into a lower recycle conduit with means for cooling thecaustic before it is transported to a destination selected from a groupconsisting of an upper recycle conduit, the lower chamber, and anoffloading conduit; introducing the gas and cooled caustic from theupper recycle conduit into the upper chamber through a mass transferapparatus that effects contact between the gas and cooled caustic andcleans the gas; allowing the caustic and cooled gas to separate in agas-liquid separation zone in the upper chamber; transporting the cleangas from the upper chamber; and transporting the caustic from the upperchamber into the upper recycle conduit.
 2. The process of claim 1,wherein the step of oxidizing the spent caustic in the lower chamberincludes introducing the spent caustic into the chamber adjacent to thebottom of the chamber.
 3. The process of claim 1, wherein the step ofoxidizing the spent caustic in the lower chamber includes introducingthe gas into the chamber to contact the spent caustic near the bottom ofthe chamber.
 4. The process of claim 1, wherein the step of oxidizingthe spent caustic in the lower chamber includes passing the caustic andgas through multiple compartments vertically superposed in the chamber,each forming an oxidation zone.
 5. The process of claim 4, wherein thenumber of compartments is four.
 6. The process of claim 4, including theadditional step of passing the gas and caustic from compartment tocompartment through a redistributor positioned in each partitionseparating two compartments.
 7. The process of claim 1, wherein the stepof oxidizing the spent caustic in the lower chamber includes theadditional step of using a flow distributor to mix the gas with thecaustic.
 8. The process of claim 1, wherein the communication of thecaustic and gas from the lower chamber to the middle chamber is througha chimney tray defining the upper limit of the lower chamber.
 9. Theprocess of claim 8, wherein the step of separating the caustic and gasin the middle chamber involves collecting the caustic in the chimneytray and collecting the gas above the caustic.
 10. The process of claim1, wherein the communication of the caustic and gas from the lowerchamber to the middle chamber is through a conduit connecting the twochambers.
 11. The process of claim 1, wherein the level of caustic inthe middle chamber is monitored by a liquid level control system thatoffloads caustic from the lower recycle conduit when the capacity of themiddle chamber is reached.
 12. The process of claim 1, wherein in thestep of communicating the gas and caustic through the mass transferapparatus, the mass transfer apparatus comprises a conduit extendinginto the upper chamber having inlets for introducing the gas and causticand an outlet for release of the gas and caustic from the conduit, saidconduit containing a plurality of fibers positioned longitudinallywithin the conduit making contact with collected caustic in a liquidcollection zone of the upper chamber.
 13. The process of claim 1,wherein the level of caustic in the upper chamber is monitored by aliquid level control system that releases caustic from the upper recycleconduit into the lower recycle conduit when the capacity of the upperchamber is reached.
 14. The process of claim 1, wherein the step oftransporting the clean gas from the upper chamber includes theadditional step of controlling the gas transport with a back pressurecontrol valve.
 15. The process of claim 1, wherein the step oftransporting the clean gas from the upper chamber includes removal ofgas from the chamber upon reaching a critical level of pressure througha second conduit controlled by a pressure relief valve.
 16. The processof claim 1, wherein the step of transporting the clean gas from theupper chamber includes communicating gas through a demister pad.
 17. Theprocess of claim 1, wherein the means for cooling the caustic in theupper and lower recycle conduits includes passing the caustic throughcaustic/cooling water exchangers in communication with each recycleconduit.
 18. The process of claim 1, wherein the step of oxidizing thecaustic in the lower chamber includes the additional step of introducingcooled caustic into the chamber from a second conduit communicating withthe chamber when the caustic and gas reach a critical temperature thatopens a temperature control valve.
 19. In the process of oxidizingindustrial spent caustic in a processing tower of the type wherein anoxidation zone chamber provides contact between the spent caustic andoxidizing gas, the improvement comprising:introducing spent caustic intoa lower chamber of a unitary processing tower adjacent to the bottom ofthe chamber; introducing oxidizing gas into the lower chamber to contactthe caustic near the bottom of the chamber using a flow distributor tomix the gas with the caustic; allowing the spent caustic to oxidize inthe lower chamber, said lower chamber having four compartmentsvertically superposed in the chamber; passing the gas and caustic fromcompartment to compartment through a redistributor positioned in eachpartition separating two compartments; introducing cooled caustic to thelower chamber from a second conduit communicating with the chamber whenthe caustic and gas reach a critical temperature that activates atemperature control valve; communicating the caustic and gas from anupper compartment of the lower chamber to a middle chamber through achimney tray defining the upper limit of the lower chamber; separatingthe caustic and gas in the middle chamber, wherein the caustic collectsin the chimney tray and the gas collects above the caustic; transportingthe gas from the middle chamber through a conduit to an upper chamber;transporting the caustic from the middle chamber into a lower recycleconduit with two caustic/cooling water exchangers for cooling thecaustic before it is transported to a destination selected from a groupconsisting of an upper recycle conduit, the lower chamber, and anoffloading conduit; introducing cooled caustic from the upper recycleconduit and the gas from the middle chamber into the upper chamberthrough a mass transfer apparatus, said mass transfer apparatuscomprising a conduit extending into the upper chamber having inlets forintroducing the gas and caustic and an outlet for release of the gas andcaustic from the conduit, said conduit containing a plurality of fiberspositioned longitudinally within the conduit making contact withcollected caustic in a liquid collection zone of the upper chamber,effecting contact between the gas and cooled caustic waste and cleaningthe gas; allowing the caustic and cooled gas to separate in a gas-liquidseparation zone in the upper chamber; transporting the clean gas througha demister pad and from the upper chamber into a gas conduit controlledby a back pressure control valve, wherein a second conduit controlled bya pressure relief valve provides a second route for transporting gasfrom the upper chamber in over-pressure situations; and transporting thecaustic from the upper chamber into the upper recycle conduit.
 20. Inthe process of oxidizing industrial spent caustic in a processing towerof the type wherein an oxidation zone chamber provides contact betweenthe spent caustic and an oxidizing gas, the improvementcomprising:oxidizing the spent caustic in a lower chamber of a unitaryprocessing tower providing an oxidation zone wherein the spent causticis contacted with an oxidizing gas; communicating the caustic and gasfrom the lower chamber to a middle chamber; separating the spent causticand gas in the middle chamber, wherein the caustic collects in thebottom of the chamber and the gas collects above the caustic;transporting the gas from the middle chamber through a conduit to anupper chamber; transporting the caustic from the middle chamber into arecycle conduit with means for cooling the caustic before it istransported to a destination selected from a group consisting of theupper chamber, the lower chamber, and an offloading conduit; introducingthe gas and cooled caustic from the recycle conduit into the upperchamber through a mass transfer apparatus that effects contact betweenthe gas and cooled caustic and cleans the gas; allowing the caustic andcooled gas to separate in a gas-liquid separation zone in the upperchamber; transporting the clean gas from the upper chamber; andtransporting the caustic from the upper chamber into the recycleconduit.